Loss on drying (LOD) testing is a core quality-control parameter for pesticide technical materials (TC), quantifying the moisture and volatile matter released when a sample is heated under precisely specified conditions. This article explains the testing principle, applicable sample types, test items, gravimetric and instrumental methods, related GB/T, CIPAC, ISO and ASTM standards, and typical industrial applications of LOD testing for pesticide active ingredients. It is written for formulators, QC chemists, regulatory specialists, and procurement teams who need reliable data to verify purity, storage stability, and specification compliance.
Technical Overview
Loss on drying (LOD) is a gravimetric test that measures the percentage of mass a sample loses when it is dried under controlled conditions of temperature, time, and pressure. For pesticide technical materials (TC, also called technical-grade active ingredients or TGAI), the mass loss is attributed primarily to free water, together with residual synthesis solvents and other low-boiling volatile impurities.
LOD is critical for pesticide technical materials for several reasons. First, technical materials are high-purity products, typically assayed at 95% to 98% active content or above, so even 1% of moisture measurably dilutes the effective active ingredient and affects the calculation of active content on a dry basis. Second, moisture drives hydrolytic degradation of ester-type actives, such as many organophosphate and pyrethroid insecticides, reducing shelf life. Third, excess moisture causes caking of powder-form technical materials, interferes with downstream formulation processes such as wettable powders, water-dispersible granules, and emulsifiable concentrates, and may promote corrosion of packaging.
Because of these effects, product specifications established under FAO/WHO pesticide specifications, national GB standards, and buyer contracts almost always define a maximum permissible loss on drying, commonly in the range of 0.5% to 2.0% depending on the active substance. Accredited third-party laboratories routinely perform LOD testing to support batch release, registration, and international trade.
Testing Samples
Loss on drying testing applies to technical-grade active ingredients and technical concentrates (TK) in all common physical forms, including crystalline powders, granules, flakes, pastes, waxy solids, viscous melts, and low-melting solid technical materials.
Insecticide Technical Materials
Representative insecticide technical materials submitted for LOD testing include organophosphates such as chlorpyrifos, dichlorvos, and phoxim; pyrethroids such as lambda-cyhalothrin, beta-cypermethrin, and permethrin; neonicotinoids such as imidacloprid and thiamethoxam; and other classes such as abamectin, chlorantraniliprole, and buprofezin. Low-melting pyrethroids and hydrolysis-sensitive organophosphates require carefully selected, milder drying conditions.
Herbicide Technical Materials
Common herbicide technical materials include glyphosate and its salts, atrazine, acetochlor, butachlor, paraquat, nicosulfuron, and pretilachlor. Water-soluble salts and hygroscopic amine salts demand rapid, humidity-controlled sample handling to avoid moisture pickup during weighing.
Fungicide Technical Materials
Fungicide technical materials such as carbendazim, mancozeb, azoxystrobin, tebuconazole, difenoconazole, and sulfur are frequently tested. Dithiocarbamates such as mancozeb are thermally sensitive, so reduced-temperature or vacuum drying is specified instead of prolonged heating at 100 degrees Celsius or above.
Other Active Substances and Intermediates
The same LOD procedures extend to plant growth regulators such as gibberellic acid and ethephon, synergists such as piperonyl butoxide, rodenticide technical materials, and pesticide synthesis intermediates, where volatile solvent residues directly influence quality assessment.
Testing Items
The scope of loss on drying testing for pesticide technical materials typically covers the following items:
- Loss on drying at atmospheric pressure under specified temperature and time, for example 100 to 105 degrees Celsius for 1 to 3 hours
- Loss on drying under reduced pressure, for example 60 to 70 degrees Celsius at 5 kPa or below, for thermolabile or low-melting technical materials
- Loss on drying under inert gas protection for oxidation-sensitive active substances
- Confirmatory water content determination by Karl Fischer titration where differentiation between true moisture and other volatiles is required
- Verification of drying to constant mass and assessment of replicate precision between duplicate determinations
- Conversion of active ingredient assay results to a dry or as-received basis using the measured LOD value
Results are reported as percentage of mass fraction (m/m), normally to two decimal places, together with the exact drying temperature, duration, and pressure applied, because these conditions are integral to the validity and comparability of the result.
Testing Methods
Gravimetric Oven-Drying Method
The reference approach is the classical gravimetric method described in standards such as GB/T 6284 for determination of water content by loss of mass on drying in chemical products, and in the CIPAC physical-chemical test methods for pesticides. A clean weighing bottle with lid is dried to constant mass and cooled in a desiccator. A test portion, typically 1 to 2 g weighed to 0.1 mg, is spread in a thin, even layer, and the open bottle is placed in a temperature-controlled drying oven at the specified temperature, commonly 100 to 105 degrees Celsius, for the specified time. The bottle is capped, cooled in a desiccator for about 30 minutes, and reweighed. Drying, cooling, and weighing cycles are repeated until constant mass is achieved, typically when the difference between consecutive weighings does not exceed 0.3 mg. Loss on drying is calculated as the mass loss divided by the initial sample mass, multiplied by 100.
Vacuum and Low-Temperature Drying
For technical materials that melt, sublime, or decompose near the standard drying temperature, a vacuum oven is used, generally at 60 to 70 degrees Celsius under 5 kPa absolute pressure or lower, sometimes with a dry inert gas bleed. This approach lowers the effective evaporation temperature, allowing moisture removal without altering the active substance, and is widely applied to pyrethroids, heat-sensitive fungicides, and easily oxidized materials.
Rapid Instrumental Methods
Karl Fischer Titration as a Complementary Determination
Wherever the distinction between true water and other volatile matter matters, volumetric or coulometric Karl Fischer titration is performed in parallel, following ISO 760 for the general Karl Fischer method, ASTM E203, and GB/T 1600 for the determination of water in pesticides. Comparing the Karl Fischer water value with the total LOD reveals whether residual solvents contribute significantly to the mass loss, which is essential for diagnosing the cause of out-of-specification results.
Quality Control and Result Interpretation
- Analytical balances must be calibrated and traceable, and ovens should be temperature-mapped to confirm uniformity at the set point.
- Duplicate determinations should agree within the repeatability limit defined by the method, typically 0.05% absolute or better for well-behaved powders.
- Hygroscopic technical materials must be sampled and weighed quickly, or handled in a dry-box, and desiccators must be charged with active, regenerated desiccant.
- Any discoloration, melting, or caking observed after drying must be recorded, because it signals thermal alteration and may invalidate the LOD result.
Applications
- Batch release quality control at technical material manufacturing plants, confirming compliance with in-house and contractual LOD limits before shipment
- Preparation of pesticide registration dossiers and demonstration of conformity with FAO/WHO specifications and relevant national GB product standards
- Import and export inspection, customs clearance support, and settlement of trade disputes through testing at accredited third-party laboratories
- Correct calculation of active ingredient content on a dry basis for formulation design, dosing, and declared active content on finished product labels
- Accelerated and long-term storage stability studies, where increasing LOD tracks moisture ingress and correlates with degradation of the active substance
- Root-cause investigation of manufacturing problems such as powder caking, reduced potency, hydrolytic breakdown, and poor dispersibility in downstream formulations
- Supplier qualification and periodic surveillance programs operated by formulators and distributors
FAQ
What is the difference between loss on drying and water content?
Loss on drying measures the total mass lost on heating, which includes water plus residual solvents and other volatile impurities. Karl Fischer water content measures only water through a specific chemical reaction. LOD is therefore equal to or higher than the Karl Fischer value, and the difference indicates the contribution of non-aqueous volatiles.
How are the drying conditions selected for a specific technical material?
Conditions are chosen based on the thermal stability and melting point of the active substance and are fixed in the product specification, monograph, or agreed test protocol. Stable crystalline materials are usually dried at 100 to 105 degrees Celsius, while low-melting or heat-sensitive materials are dried at 60 to 70 degrees Celsius under vacuum.
What are typical acceptance criteria for loss on drying?
Limits depend on the active substance and the applicable specification. Crystalline technical materials commonly carry limits of 0.5% or 1.0% m/m, while hygroscopic salts and certain liquid or paste technical materials may be permitted up to 2.0% or higher. The contract or registered specification always prevails.
Can a rapid moisture analyzer replace the oven method for release testing?
Only if the instrument method has been validated against the reference gravimetric method for the specific technical material, with demonstrated equivalence, and the validated program is strictly followed. Unvalidated rapid readings are suitable for process monitoring but not for formal release decisions.
Why did our sample lose more mass than expected without high water content?
When LOD significantly exceeds the Karl Fischer water value, residual process solvents, low-boiling impurities, or partial decomposition or sublimation during drying are likely contributors. A Karl Fischer cross-check and a review of the drying temperature against the thermal behavior of the active substance are recommended.
What precautions apply to hygroscopic or low-melting technical materials?
Hygroscopic samples should be homogenized rapidly in a low-humidity environment, weighed into pre-tared vessels without delay, and cooled in a fully effective desiccator. Low-melting materials should be dried under vacuum at reduced temperature so that the sample never reaches its melting or softening point during the test.